Studies are being carried out to elucidate the factors which regulate the biosynthesis of polyunsaturated fatty acids in mammalian systems. These studies have involved the total organic synthesis of a variety of radioactive and nonradioactive unsaturated fatty acids. Feeding studies have been used to determine how various unsaturated fatty acids are metabolized. Competitive feeding studies have been carried out in attempts to determine whether certain acids may influence the metabolism of other acids. Enzyme studies have been carried out on the microsomal desaturation and chain elongation of a variety of different radioactive acids. These studies have utilized microsomes from rats maintained on a variety of different dietary conditions. These studies have provided information as to the type of structure necessary in order to serve as a substrate for desaturation and chain elongation and also suggest the presence of more than one desaturate and more than one chain elongating enzyme in the microsome.